Noninvasive ultrasonic proximity detector for a fluid actuated cylinder

ABSTRACT

The invention provides a generally noninvasive device and method for detecting a reciprocative piston located within a cylinder bore. An ultrasonic transducer mounted on the outside of the cylinder wall selectively generates an ultrasonic transmission signal directed through the cylinder. The transducer then receives any desired ultrasonic signal produced by propagation of the ultrasonic transmission signal. Control circuitry processes electrical signals produced on the ultrasonic transducer by the propagated ultrasonic signal to indicate presence or absence of the piston. 
     In presently preferred embodiments, the control circuitry determines whether the piston is present by comparing a signal level in the received ultrasonic signal during a sample time window in which the desired signal may be expected with a predetermined threshold level. To perform this function, electrical signals produced on the transducer by the propagated ultrasonic signal are first passed to an amplifier having a selected gain. An output of the amplifier is then fed to an detector circuit which produces an detection level signal which may be representative of energy present in the received ultrasonic signal during the sample time window. Comparator means then perform the comparison of the detection signal with the threshold. 
     In many embodiments, it is generally desirable to utilize two piston sensors respectively located to indicate presence of the piston at limits of its reciprocative stroke. For greatest flexibility, some presently preferred embodiments of the device are capable after being initially activated to identify with which cylinder model, from a plurality of such models, the detector is being used. In this way, the device can automatically set certain operating parameters to optimize signal-to-noise ratio. The device may also incorporate light emitting diodes or other indicators to alert the user of current operating conditions.

This application is a continuation of application Ser. No. 08/027,370,filed Mar. 8,1993, pending.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a proximity detector utilized to sensethe presence of a piston reciprocatively mounted within a cylinder bore.More particularly, the invention relates to a device and method forproviding such a proximity detector which is generally noninvasive ofthe cylinder housing assembly.

2. Description of the Prior Art

In the operation and control of fluid actuated cylinders, proximitydetector devices are often utilized to indicate presence of the movingpiston therein. Frequently, such devices are employed to detect thepiston at respective terminal limits of its reciprocative stroke. In amanufacturing environment, for example, this information can beadvantageously used to maintain efficient workpiece flow.

Prior art proximity detector devices have generally requiredmodification of the cylinder housing assembly. For example, somecommercially available proximity detectors utilize a magnetic detectormounted externally of the cylinder assembly to detect a magnetic memberattached to the piston. Passage of the magnetic member causes activationof the detector, thus affirmatively indicating presence of the piston.Generally, however, such devices are limited to applications in whichthe cylinder is constructed of a nonmagnetic material such as aluminum.Hydraulically actuated cylinders are typically made of steel, whichinhibits the external detection of a magnetic field. Other proximitydetector devices, equally useable with pneumatically and hydraulicallydriven cylinders, require an even greater degree of invasivemodification to the cylinder assembly.

The phenomena of ultrasonic energy propagation has been utilized in anumber of applications. For example, nondestructive testing (NDT)techniques often utilize ultrasonic pulses to detect flaws in metal andweld joints. Ultrasonic energy propagation has also been used to measurethe flow of fluid in a conduit. Furthermore, various devices have beenconstructed wherein distance to an object or thickness of a workpiece ismeasured by the duration between transmission of an ultrasonic pulse andreception of the subsequent echo.

One such device is used to measure the position of a piston movingwithin a hydraulic cylinder. The ultrasonic transducer in this device isinvasively mounted to the end cap of the cylinder. An ultrasonic pulseis transmitted into the hydraulic fluid coaxial to movement of thepiston. An echo is reflected from the face of the piston and returns tothe transducer after a duration proportional to the distance from thetransducer.

SUMMARY OF THE INVENTION

The invention provides a generally noninvasive device and method fordetecting a reciprocative piston located within a cylinder bore. Thedevice includes transmitter means mounted on the outside of the cylinderwall to selectively generate an ultrasonic transmission signal directedthrough the cylinder wall into the cylinder bore. Receiver means alsomounted on the cylinder wall receive any desired ultrasonic signalresulting from propagation of the transmitted signal through at leastpart of the cylinder assembly. This received signal is converted to anelectrical signal which is processed by control means to indicatepresence or absence of the piston.

In some presently preferred embodiments, the transmitter means andreceiver means may comprise a single ultrasonic transducer incorporatedwithin a piston sensor positionable on the outside of the cylinder wall.In many applications it may be desirable to position two such sensorssuch that the piston may be detected at respective limits of itsreciprocative stroke. The transducer of each piston sensor wouldselectively emit an ultrasonic transmission signal into the cylinderassembly. An ultrasonic echo signal later returns to the transducer andconverted to an electrical echo signal. This electrical echo signalcontains information which can be processed by the control means toindicate presence or absence of the piston.

The control means in presently preferred embodiments is operative todetermine whether the piston is present adjacent the transducer bycomparing a selected threshold level with a signal level in theultrasonic echo signal during a selected sample time window. This may beaccomplished by first passing the electrical echo signal to amplifiermeans having a selected gain. An output of the amplifier means is thenfed to a signal level detection means which produces a detection signalrepresentative of energy present in the ultrasonic echo signal duringthe sample time window. Comparator means perform the comparison of thedetection signal with the selected threshold level.

To enhance immunity from noise and other extraneous influences, a numberof such detection signals may be produced from a corresponding number ofrespective ultrasonic transmission signals generated in rapidsuccession. These detection signals may then be averaged or otherwiseevaluated before comparison with the threshold level. A degree ofhysteresis may be provided by utilizing a low threshold level foraffirmative indication that the piston is not present adjacent thetransducer. In this case, a piston would be indicated as absent only ifthe value of the detection signal is below the low threshold level. Adetection signal between the high and low thresholds could beinterpreted to not affect the prevailing state.

while the device may be dedicated to a particular cylinder model,greater flexibility is achieved if adaptability for use with any one ofa number of different sized power actuated cylinder models isfacilitated. In presently preferred embodiments, this capability may beautomatically provided so long as the respective cylinder models have atleast one recognizable ultrasonic propagation characteristic. Typically,this recognizable characteristic may be a particular time-of-flightbetween generation of an ultrasonic transmission signal and firstreceipt of the corresponding echo. The control means may derive thistime-of-flight measurement by comparing a preselected threshold levelwith energy levels in sequentially produced ultrasonic echo signalsduring corresponding of successively delayed sample time windows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevation of a proximity detector device constructed inaccordance with the invention having two piston sensors positioned on acylinder housing shown in partial section to illustrate thereciprocative piston therein.

FIG. 1A is an enlarged fragmentary view illustrating one of the pistonsensors of FIG. 1 partially cut away to reveal an ultrasonic transducertherein.

FIGS. 2A and 2B are a fragmentary diagrammatic elevation anddiagrammatic end view, respectively, illustrating transmission andreflection of ultrasonic energy within a power actuated cylinder.

FIGS. 3A ant 3B are graphical plots illustrating various signalwaveforms produced when the piston is respectively absent and presentadjacent the piston sensor.

FIG. 4 is a diagrammatic representation illustrating ultrasonicpropagation characteristics resulting when the piston is modified byhaving an annular groove formed about the circumference thereof.

FIG. 5 is a diagrammatic representation illustrating propagation ofultrasonic energy when a pair of ultrasonic transducers (T and R) areutilized with each piston sensor.

FIG. 6 is a graphical plot illustrating signal waveform conditionsresulting from the configuration of FIG. 5 in the piston absent andpiston present states, respectively.

FIG. 7 is a perspective view of a plurality of differently sized poweractuated cylinder models.

FIGS. 8A through 8C are graphical plots of signal waveforms produced inrespective of the cylinder models of FIG. 8 in the piston absent state.

FIGS. 8D through 8F are graphical plots of signal waveforms produced inrespective of the power actuated cylinder models of FIG. 8 during thepiston present state.

FIG. 9 is a schematic representation of a presently preferred embodimentof a circuit for the proximity detector device of the invention.

FIG. 10 is a block flow diagram of control software implemented toeffect various functions which may be performed by a presently preferredembodiment of the proximity detector device of the invention.

FIG. 11 is a block flow diagram of the cylinder identify subroutineshown generally in FIG. 10.

FIG. 12 is a block flow diagram generally illustrating the four testsshown generally in FIG. 10.

FIG. 13 illustrates a lookup table containing characteristic andparameter information utilized by the four tests shown generally in FIG.10.

DETAILED DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENTS

FIG. 1 illustrates a presently preferred embodiment of the proximitydetector of the invention utilized in conjunction with a typical poweractuated cylinder assembly. The cylinder assembly generally comprises acylinder wall 10 closed at each end by end caps 11 and 12. End caps 11and 12 are maintained securely in position by a number of tie rods orbolts, such as tie rods 13 and 14. Inner surface 15 of cylinder wall 10defines a cylinder bore within which piston 16 is located. Piston 16,which is attached to piston rod 17, is movable within the cylinder borebetween respective terminal limits of a reciprocative stroke.

in this case, piston 16 is actuated by fluid, such as hydraulic fluid,passed into the cylinder housing through fluid passages 18 and 19.Specifically, ingress of fluid into the cylinder bore through fluidpassage 18 (as shown by arrow "x") fills an expandable fluid chamber infront of piston 16. This causes piston 16 to move to the right.Similarly, piston 16 may be moved to the left by ingress of fluidthrough fluid passage 19 (as shown by arrow "y"). Typically, piston 16will be equipped with seal rings, such as rings 20 and 21, to preventfluid from escaping the respective expandable fluid chambers. Piston 16may also include a wear ring 22 to minimize frictional wear to surface15.

In presently preferred embodiments, the proximity detector deviceincludes a pair of piston sensors 25 and 26 respectively positioned todetect piston 16 at or near the terminal limits of its reciprocativestroke. In other embodiments, one sensor or a plurality of sensors canbe mounted where it is desirable to detect the presence or absence of amovable member, such as piston 16. Sensors 25 and 26 are mounted on theoutside surface 27 of cylinder wall 10 and may be attached in anyappropriate manner which provides secure placement. Typically, suchattachment may be effected by clamping piston sensors 25 and 26 to tierod 13. Electrical signals from sensors 25 and 26 are passed viainterconnecting cable 28 to control circuitry within housing 29. Thecontrol circuitry processes these electrical signals to determine thepresence or absence of piston 16 adjacent the respective of sensors 25or 26. The control circuitry may then produce control signals indicatingthe state of piston 16 which may be passed to external controllers orthe like via output line 30. Although housing 29 is shown as not beingattached to the cylinder assembly, the control circuitry therein may beminiaturized, such as by using application specific integrated circuits(ASICs), to facilitate such attachment.

The device may include various user interface features to provide readyindication of current operating conditions. For example, piston sensors25 and 26 may include respective light emitting diodes (LED) 31a and 31bof a first color, such as red, indicating that piston 16 is presentthereunder. Similarly, LEDs 32a and 32b having a second color, such asyellow, may also be provided to indicate that secure placement of therespective of piston sensor 25 or 26 to cylinder wall 10 cannot beconfirmed. Thus, the operator can be alerted that attention is required.Similarly, housing 29 may have red LEDs 33a and 33b respectivelycorresponding to LEDs 31a and 31b. A yellow LED 34 may also be providedon housing unit 29 to alert the operator that one of piston sensors 25and 26 is not properly attached to cylinder wall 10. Additionally, theinvention offers the capability of utilizing an infrared LED 35 toprovide interface with a wireless printer, such as the Hewlett PackardHP 82240B, which receives an infrared input. In this way, printedinformation, such as whether the detector device is operating properly,the number of cylinder operations left before service is required, andlocal product distributors (name, address and phone number), may bereadily provided to the operator.

It can thus be seen that the presence of piston 16 is detected throughcylinder wall 10 by piston sensors 25 and 26. This is accomplishedaccording so the invention using high frequency energy propagationthrough "particle displacement" referred to more simply as "ultrasound."Specifically, piston sensors 25 and 26 generate a short ultrasonicsignal which is directed through the cylinder wall 10 into the cylinderbore. This ultrasonic signal echoes throughout the cylinder. Eachacoustic impedance change in the transmission path transforms andchanges the ultrasonic signal. Eventually, some of the transmittedenergy is received by the piston sensor and may be converted into anelectrical echo signal. Cases in which piston 16 is present or absentbeneath the respective of piston sensors 25 and 26 will producedifferent electrical signals which can be distinguished to indicate themeasured state.

An ultrasonic transducer which may be used to generate the ultrasonictransmission signal and receive the subsequent echoes is illustrated inFIG. 1A. Although FIG. 1A shows only piston sensor 25, piston sensor 26may be similarly constructed. The ultrasonic transducer may comprise apiezoelectric crystal 40 sandwiched between electrical contacts 41 and42. A couplant layer 43 is provided between contact 42 and the outsidesurface 27 of cylinder wall 10 to facilitate effective ultrasonictransmission. Couplant layer 43 may be easily formed using anappropriate gelatinous substance. It is recommended that such asubstance be nonreactive and preferably water based, such as Ultragel IIcouplant. In situations in which a generally greater degree of couplantloss may be tolerated, couplant layer 43 may be permanently formed usinga thin film of polymeric material. A damping material 44 behind contact41 absorbs unwanted ultrasonic energy transmitted back into sensor 25. Aspring 45 urges the ultrasonic transducer into secure engagement againstoutside surface 27.

Referring to FIG. 2A, the propagation characteristics of an ultrasonictransmission signal 50 such as may be generated by ultrasonictransducers 25 and 26 is illustrated. For purposes of illustration only,the various remainder and echo signals that will be described are shownas laterally displaced. Propagating energy from signal 50 will in thiscase encounter two significant interfaces which will influence themagnitude of energy subsequently reflected back to the transducer.Specifically, signal 50 first travels a distance W equal to the width ofcylinder wall 10. There it encounters an interface defined by theboundary between cylinder wall 10 and the gap between wall 10 and piston16. At this wall-gap interface, most of the energy originally containedin signal 50 is reflected back towards to the transducer, as shown byecho 51. The remainder 52 of the energy in signal 50 will continue intothe gap.

After traveling a distance G equal to the gap between piston 16 and wall10, the second interface, defined by the boundary between the gap andpiston 16, will be encountered. At this gap-piston interface, most ofthe energy in remainder 52 will again be reflected, as shown by echo 53.A remainder 54 of the energy in remainder 52 will continue into piston16. After traveling distance G, echo 53 will encounter the wall-gapinterface where, as shown by echo 55, most energy will again bereflected. A remainder 56 of echo 53, however, will continue intocylinder wall 10. As can thus be seen, remainder 56 will only resultwhen piston 16 is in such a position that energy from signal 50 may bereflected. The control circuitry of the device interprets the totalultrasonic echo signal to determine whether echo 56 is present.

Assuming that cylinder wall 10 and piston 16 are constructed of metalsuch as steel, and the wall to piston gap is filled with hydraulicfluid, the expected energy of echo 56 may be calculated. Specifically,the percentage of energy in each echo may be found using the followingequation:

    Reflection=[(Z.sub.1 -Z.sub.2)/(Z.sub.1 +Z.sub.2)].sup.2 ×100%

where Z₁ is the acoustic impedance in the material through which theenergy is first propagating and Z₂ is the acoustic impedance of thematerial on the other side of the Interface. Using a figure of 4.56×10⁶as the acoustic impedance of steel and 0.128×10⁶ as the acousticimpedance of hydraulic fluid yields the following: (1) signal 50=100%energy; (2) echo 51=89.4% energy; (3) remainder 52=10.6% energy; (4)echo 53=9.5% energy; (5) remainder 54=1.1% energy; (6) echo 55=8.496%energy; and (7) remainder 56=1.004% energy. This calculation ignores anyfocusing or scattering effects due to the curvature of the respectiveinterfaces. Referring to the end view of FIG. 2B, it becomes apparentthat these effects may tend to scatter energy away from the transducer.As a result, it may be proper to further reduce the estimated energy ofecho 56 to below this value of approximately 1%.

using these calculations only, it is dubious that sufficientsignal-to-noise ratio (SNR) could be provided for many practicalapplications. As a result, it would be difficult to distinguish using a1% detection signal between the piston present and piston absent states.According to the invention, however, SNRs greater than 20db have beenexperimentally obtained. This represents echo energy having amplitudesup to ten times or more that of errant noise. Such SNRs provide signalswhich are easily detectable in most practical applications. Theseresults are believed to be obtained by the frequency of the ultrasonictransmission signal experiencing a degree of resonance in the gapbetween wall 10 and piston 16. The mean value of this gap is frequentlygiven as a production specification for the particular cylinder model. Arange within which the value of this gap may typically vary isapproximately 0.003 inches to 0.023 inches. The cylinder wall thicknessmay typically vary within a range of approximately 0.1170 inches to 1.04inches.

Resonance may best be obtained if the wavelength of the chosen frequencyof signal 50 is approximately four times the mean gap width G. It isalso important, however, that the fundamental frequency of signal 50have significant wavelength with respect to the cylinder wall. Thus, inpresently preferred embodiments the wavelength of the chosen frequencyshould be no more than one-tenth the width W of cylinder wall 10. Otherfactors, which may be more significant in some embodiments than inothers, also influence the choice of frequency. For example, it isdesirable to use higher frequencies to facilitate damping within thepiston sensor itself, thus contributing to an overall reduction in size.At very high frequencies, however, a piezoelectric transducer willbecome so small that it may be difficult to manufacture. For mostexpected applications of the device, the resulting ideal frequency willgenerally lie somewhere within the range of 1-25 MHz. Generally, thesignal frequency may be expected to fall within the range of 8-12 MHz.

Signal-to-noise characteristics may be improved in some cases by formingthe ultrasonic transmission signal of a pulse train of driving pulses.The number of driving pulses is typically derived experimentally for theparticular cylinder model. With various cylinder models, suitable SNRsmay be obtained with the number of driving pulses generally ranging fromone to seven. The particular number within this range generally dependson the specific cylinder model.

Some of the distinguishable signal characteristics between the pistonabsent and piston present states are illustrated in FIGS. 3A and 3B.Referring particularly to FIG. 3A, trace A1 illustrates the electricalsignal across the ultrasonic transducer in the piston absent condition.Trace A1 can be best understood if it is divided into a number ofdistinct time periods. The plot occurring during period "a" indicatesthe ultrasonic transmission signal generated by the transducer as wellas a number of subsequent echoes occurring within the body of the pistonsensor itself. The transducer then experiences a quiet time duringperiod "b" in which the ultrasonic transmission signal is propagatingthrough the cylinder wall. After reflection from the wall-gap interface,some of the energy from the ultrasonic transmission signal returns inthe form of an initial echo. The quiet time during period "b" is thus anindication of the time-of-flight of the ultrasonic transmission signalin the cylinder wall, which is itself indicative of cylinder wallthickness. The initial echo will excite the transducer for a period "c"after which will occur a second quiet time during period "d". A secondecho (partially shown) is received by the transducer during period "e".

Referring particularly to FIG. 3B, it can be seen that, as expected,trace P1 is similar to trace A1 during periods "a", "b" and "c". Duringperiod "d", however, the received signal of trace P1 is significantlyhigher than in trace A1. This energy slowly dissipates until receptionof the second echo at the beginning of period "e". This signaldifference during period "d" is evidently due to reflection from theouter surface of the piston. It is especially noteworthy that,particularly immediately after period "c", the signal level issignificantly greater than the 1% return predicted by the analysisaccompanying FIG. 2A. This relatively high signal level is easilydetectable by the control circuitry.

To detect the signal reflected from the piston, presently preferredembodiments of the invention utilize a sample time window technique.Specifically, a sample time window is synchronized to begin at aselected start time after generation of the ultrasonic transmissionsignal. This sample time window remains "open" for a selected durationwithin which a piston detection signal is derived. In presentlypreferred embodiments, the sample time interval, or "window," issynchronized to derive the piston detection signal during the time inwhich energy reflected from the outer surface of the piston is expected.For example, referring to FIG. 3A, a sample time window 60 isillustrated by the trace immediately below trace A1. Window 60 has astart time corresponding to the beginning of period "d" and continuesfor a short duration thereafter. A similar sample time window 61 isillustrated in FIG. 3B in the trace immediately below trace P1.

The detection signals thus derived are shown in traces DS1 and DS2 ofFIGS. 3A and 3B, respectively. In the piston absent condition, trace DS1maintains a nearly constant level throughout the display. As shown intrace DS2, however, the detection signal will rise to a significantlyvalue in the piston present state where it may be maintained by thecontrol circuitry. By comparing the final energy level signal at period"e" with a selected threshold level, a decision can be made as towhether a piston present or present absent state prevails.

In certain situations, the gap width G between the cylinder wall and thepiston outer surface may be smaller than necessary to achieve properresonance. For example, a heavy side load on a horizontally mountedpiston may cause an otherwise sufficient gap width to become very smallat the top surface of the piston. In this case, it may be possible tosimply mount the piston sensor on the bottom or side of the cylinderassembly where gap widths should remain of a sufficient value.

In other cases, such as when the cylinder is manufactured having gapwidth insufficient for resonance, a slight modification of the signalprocessing techniques may be required. The piston, however, may bedetected according to the teachings of the invention even in thecomplete absence of an identifiable gap between the piston and thecylinder wall. For example, referring again to FIG. 2A, a gap width G ofzero will cause a measurable reduction in the energy of echo 51. Inother words, a greater percentage of ultrasonic transmission signal 50will continue into piston 16. As a result, the amplitude during period"c" in a trace of the electrical signal produced at the ultrasonictransducer will be less than that of the piston absent state shown intrace A1. To measure this lower energy level, the sample time window maybe synchronized to occur during period "c." The detection signalproduced during this sample time window can then be compared with apreselected threshold level. If the detection signal is less than thisthreshold, the piston present state can be deemed to exist.

Referring now to FIGS. 4 through 6, alternative embodiments of certainaspects of the invention will be illustrated which may have utility inspecific applications. In some applications, for example, it may bedesirable to modify the piston itself such that a resonant cavity iscontinually maintained. A presently preferred technique for maintainingsuch resonance is illustrated in FIG. 4. An annular groove 65 formed inpiston 66 maintains a resonant cavity at the interface of cylinder wall67 and the hydraulic fluid even in the event that the gap width G isreduced to zero. Instead of a simple V-shape, groove 65 employs acomplex configuration of multiple stepped surfaces 68 which are slopedupward to permit a number of path lengths for the reflected ultrasonicenergy. A simple "V" groove, on the other hand, would permit only onesuch path length. As a result, the configuration of groove 65 permitsresonance to occur at a wider range of frequencies.

Referring particularly to FIG. 5, an alternative piston sensor isillustrated utilizing a pair of transducers 70 and 71 positioned atspaced apart locations on the outside of cylinder wall 72. When one oftransducers 70 or 71 is used as a transmitter, the other is awaitingreception of the ultrasonic echo signals thus produced. For example,assume transducer 70 is functioning as a transmitter (T) and transducer71 is functioning as a receiver (R). The ultrasonic transmission signalmay be injected into cylinder wall 72 at a selected angle such thattransducer 71 will effectively receive the echo energy caused byreflection from piston 73. Traces A2 and P2 of FIG. 6 respectively showelectrical signals at the transducers in the piston absent and pistonpresent states. During periods "f", "g" and "h" traces A2 and P2 aresimilar. During period "i", however, trace P2 has a higher signal leveldue to reflection from piston 73. An interesting aspect of thisconfiguration is revealed during period "j". Other than the echoreferenced as 74, subsequent echoes are generally absent from traces A2and P2. This is because they have been reflected down the cylinder wallaway from transducer 71.

The presence of echo 74 can be advantageously used in some applicationsto detect the presence of the piston at both limits of its reciprocativestroke utilizing a single piston sensor. Specifically, the piston sensormay be placed adjacent one stroke limit to directly detect the piston asdescribed above. To determine that the piston has extended to its otherstroke limit, reflection from the piston shaft is monitored. Atextension of the piston, pressure will cause fluid behind the piston toexperience an increase in density. This increase in density results in acorresponding increase in the time of flight of echo 74, the detectionof which can serve as an indicator that the piston has reached itsstroke limit.

In order no provide greatest operative flexibility, the proximitydetector may be adaptable for use with any one of a plurality ofcylinder models. For example, power actuated cylinders are frequencyoffered by manufacturers as a line of cylinders having different boresizes. Such a line 80 of power actuated cylinders is illustrated in FIG.7 having respective cylinders 80a, 80b and 80c. While cylinders 80a, 80band 80c generally have similar ultrasonic energy propagationcharacteristics due to their similar construction, differences willgenerally exist. For example, the respective cylinders within the linemay have different cylinder wall thicknesses or mean gap widths betweenthe piston and cylinder wall. As a result, each will exhibit slightlydifferent ultrasonic energy propagation characteristics. For bestperformance, the operating parameters of the control circuitry arepreferably altered for each cylinder model with which the proximitydetector will be used. While cylinder selection to set up the operatingparameters of the control circuitry may be accomplished manually using aswitch or the like, presently preferred embodiments include means forautomatically making such selection.

The different ultrasonic propagation characteristics between cylindermodels 80a, 80b and 80c can best be described with reference to FIGS. 8Athrough 8F. Trace A3 of FIG. 8A corresponds to a piston absent conditionin cylinder 80a. Trace P3 shown in FIG. 8D corresponds to a pistonpresent state in cylinder 80a. Similarly, traces A4 and P4 respectivelyillustrated in FIGS. 8B and 8E correspond to respective piston absentand piston present states in cylinder 80b. Traces A4 and P4 of FIGS. 8Cand 8F, respectively, likewise correspond to piston absent and pistonpresent states in cylinder 80c.

FIGS. 8A through 8F clearly illustrate that, due to differences incylinder wall thickness, the quiet time (period "b") between generationof the ultrasonic transmission signal and first reception of the initialecho will be different for cylinders 80a, 80b and 80c. Thus, in each ofthese cases, the sample time window must begin at a different time withrespect to generation of the ultrasonic transmission signal.

Other operating parameters of the proximity detector may need to beadjusted for effective operation with each of cylinder models 80a, 80band 80c. For example, the driving frequency of the ultrasonictransmission signal is preferably related to the cylinder wall thicknessand mean gap width as discussed above. Thus, if these variables aredifferent in cylinder models 80a, 80b and 80c, the driving frequencyshould preferably be adjusted accordingly. Also, the particular numberof driving pulses comprising the ultrasonic transmission signal mayproduce improved signal-to-noise characteristics with respect to aspecific one of cylinder models 80a, 80b and 80c. The preferredthreshold levels used to determine presence or absence of the piston mayalso be different depending on the cylinder model with which theproximity detector is being used. Also, to optimize SNR withoutsaturation, it may be desirable for a particular cylinder model toadjust the gain by which the echo signal produced across the ultrasonictransducer is amplified.

The circuitry utilized in presently preferred embodiments of theproximity detector device is illustrated in FIG. 9. Each piston sensor25 and 26 contains a number of circuit components which cooperate withcontrol circuitry 90 (located within housing 29 of FIG. 1) to functionin the desired manner. The LEDs illustrated in FIG. 9 correspond tothose shown in FIG. 1. Piston sensor 25 contains a piezoelectrictransducer 91 coupled through transformer T1 to a preamplifier 92 and ahigh current pulse driver 93. Similarly, piston sensor 26 contains apiezoelectric transducer 94 coupled through transformer T2 throughpreamplifier 95 and high current driver 96. Transformers T1 and T2,which are preferably wound in a 1:1 ratio, provide DC isolation betweenthe respective of transducers 91 and 94 and other circuit components.Transducers 91 and 94 are preferably sized to resonate at the centerfrequency of the bandwidth over which the ultrasonic transmissionsignals are expected to vary. For example, if it is desired that thetransmission signals vary over a range of 8-12 MHz, transducers 91 and94 may be sized to be resonant at 10 MHz.

To avoid duplication of circuit components as well as to prevent gainimbalances which may occur with such duplication, the outputs ofpreamplifiers 92 and 95 are fed to respective inputs of multiplexer("MUX") 97. In presently preferred embodiments, piston sensors 25 and 26are controlled by microprocessor 98 to operate in alternating sequence.Multiplexer 97 is thus also controlled by microprocessor 98 to receiveelectrical echo signals only from the active of piston sensors 25 and26.

The output of multiplexer 97 is fed to amplifier 99. For use with a widerange of cylinders, amplifier 99 is a gain controllable amplifier("GCA") wherein the gain is selectable by microprocessor 98 depending onthe particular cylinder model with which the proximity detector deviceis being utilized. Amplifier 99 may also incorporate a bandpass filterhaving an upper cutoff frequency several megahertz higher than thedriving frequency of the transmission signal. For example, if thehighest frequency transmitted signal expected will occur at 12 MHz, athree-to-five pole 20 MHz passband edge should be sufficient.Preferably, the output of amplifier 99 is also clamped so as not toexceed a certain maximum level.

The high frequency output of amplifier 99 is then passed to signal leveldetection circuitry 100 which produces the piston detection signal.Microprocessor 98 then converts the piston detection signal to digitalformat using an internal analog-to-digital converter. Although shown asa block, circuitry 100 comprises in presently preferred embodiments anumber of circuit elements which function to Rectify, Integrate, Sampleand Hold the electrical signal across the piezoelectric transducerduring the sample time window. As such, circuitry 100 is collectivelyreferred to as ("RISH") circuitry. The start time and duration of thesample time window, which may be placed at any point after generation ofthe ultrasonic echo signal, are communicated to circuitry 100 by aninput from microprocessor 98.

The detection signal (in digital format) is compared by microprocessor98 with the selected threshold levels to produce signals at output 101indicating presence or absence of pistons 16 adjacent sensors 25 or 26.For example, output line 102a may indicate presence of piston 16adjacent piston sensor 25 with a digital high output. A digital lowoutput on line 102a will thus indicate absence of piston 16 beneathpiston sensor 25. Similarly, high or low outputs on line 102b mayindicate respective piston present or piston absent states adjacentpiston sensor 26.

Microprocessor 98 further directs operation of pulse counter 103 andvariable frequency oscillator 104. Pulse counter 103 and variablefrequency oscillator 104 together comprise transmission means whichcontrol characteristics of the ultrasonic transmission signal to providebest results with a particular cylinder model. In presently preferredembodiments, oscillator 104 comprises a phase lock loop ("PLL")frequency generator capable of generating an oscillatory output signalhaving a frequency ranging from 8.0 MHz to 12.0 MHz in 0.5 increments.Pulse counter 103 then modulates the oscillatory output signal ofoscillator 104 by an appropriate number of driving pulses. Note thatmultiplexer 97 is connected to also receive an input from variablefrequency oscillator 104. This input may be used as desired for periodiccalibrations or other diagnostic purposes.

Referring to FIG. 10, software implemented by microprocessor 98 inpresently preferred embodiments is generally illustrated. As indicatedby block 110, each software cycle is set to begin at a launch timeslightly greater than the period required to execute the software. Ateach launch time, the software alternates between piston sensors 25 and26. Thus, for example, a 10 mS launch time indicates that sensor 25 or26 will each be actuated every 20 mS. Next, as shown by block 111, thesoftware determines whether the particular cylinder model with which theproximity detector device is being used has been identified. If not, thesoftware enters a "cylinder identify" subroutine indicated generally byblock 112. After the cylinder has been identified, the cylinder identifysubroutine exits and the software awaits the next launch time.

If the cylinder has been selected, the software implements a series offour similar tests shown by blocks 116 through 119, respectively. Thefirst test is used to verify that a quiet time exists for the specificcylinder. If the quiet time cannon be verified, the cylinder selectionis cancelled. The second test measures signal strength of the returnedecho and is used to verify secure placement of the respective pistonsensor on the cylinder housing wall. Unsatisfactory results from eitherof the first two tests causes the corresponding yellow LED to beactivated. The third and fourth tests are used to determine whether thepiston is present or absent beneath the respective piston sensor. Ifeither of these tests indicates that a piston present conditionprevails, the proper indicators and outputs will be set (as showngenerally by block 120). For enhanced noise immunity, presentlypreferred embodiments require each test to achieve the same resultseveral times before a change of final state is indicated.

To achieve automatic identification of a particular cylinder from anumber of such cylinders known to the microprocessor, it is onlynecessary that the respective cylinders have one recognizable ultrasonicpropagation characteristic. In presently preferred embodiments, thischaracteristic is based on the differences in cylinder wall thicknesswhich generally appear between different cylinder models within anoverall line. Specifically, different cylinder wall thicknesses willyield a particular time-of-flight between generation of the ultrasonictransmission signal and first receipt of the initial echo. Byselectively generating a sequence of ultrasonic transmission signals andmeasuring the signal level in respective of successively delayed sampletime windows, a detailed profile of the time-of-flight characteristiccan be assembled in the microprocessor. Based on this profile, theparticular cylinder model may be identified.

Referring to FIG. 11, a presently preferred "cylinder identify"subroutine utilizing this technique is illustrated. First, as shown inblock 125, the sample time window is initially set near the time whenthe first quiet time (period "b") should begin. Next, as indicated inblock 126, an ultrasonic transmission signal ("bang") is initiated.Additionally, a counter is decremented which is initially set to thenumber of successive ultrasonic transmission signals which will begenerated to derive the time-of-flight profile. As shown in block 127,when the counter does not equal zero, the software will begin a loopsearching for the first quiet time. This is accomplished as shown inblock 128 by a comparison to determine whether the detection signal("DS") is less than a selected threshold which may be referred to as acylinder identify low level ("CIL"). If the detection signal is not lessthan level CIL, then the loop goes back to block 126 for generation ofanother ultrasonic transmission signal. At the same time, the counterwill be decremented. If the counter reaches zero without the quiet timebeing verified, the subroutine exits. This condition may exist, forexample, if too much couplant is applied to the piston sensor.

Referring again to block 128, verification of the quiet time will begina second loop to identify first receipt ("rising edge") of the initialultrasonic echo. As shown in block 129, an ultrasonic echo signal isagain generated and the counter is decremented. If the counter reacheszero without the rising edge having been identified, then the subroutineexits as shown by block 130. If, however, the counter does not equalzero then the detection signal is compared with a threshold which may bereferred to as a cylinder identify high level ("CIH"). As shown in block131, if the detection signal is not greater than level CIH, then theloop is sequentially repeated until such is the case. When the detectionsignal exceeds level CIH, the time-of-flight between generation of theultrasonic transmission signal and first receipt of the initial echowill be determined. As indicated by block 132, the subroutine uses thisinformation to determine the particular cylinder model.

FIG. 12 shows a block flow diagram which is generally illustrative ofall four tests performed by microprocessor 98 after the cylinderselection has been accomplished. For each cylinder model, each test hasa dedicated look-up table containing the respective threshold values andthe like which are to be used by that test. Thus, in presently preferredembodiments, the total number of look-up tables stored in the devicewill be equal to four times the total number of cylinder models forwhich the device is useable. FIG. 13 illustrates generally such alook-up table 135 containing different characteristics or parameters ineach of a plurality of memory locations. In many applications, it willbe desirable to permanently set the look-up table at the time ofmanufacture using read only memory ("ROM"). For greatest flexibility,however, programmable read only memories ("PROM") can be used, thusallowing subsequent alteration of the respective look-up table values.

Referring again to FIG. 12, it is shown in block 140 that the first stepperformed by each of the four tests is the loading of the appropriatelook up table. Also as shown in block 140, the test next performs an"acquisition sequence." In presently preferred embodiments, theacquisition sequence comprises a number of detection signalsrespectively produced from successive ultrasonic transmission signals.The sum of these detection signals may be divided by the number ofultrasonic transmission signals in the acquisition sequence to arrive atan average value. This average value may be referred to as total sum("TS"). The acquisition sequence therefore provides a degree of immunityfrom errant noise or other extraneous influences within could have agreater effect on a single piston detection signal.

Next, as shown in block 141, total sum TS is compared with the selectedthreshold high level ("TH") for the cylinder model. As shown in block142, a value of total sum TS greater than threshold high level TH willcause a new state high to be indicated. If, however, total sum TS is notgreater than the threshold high level TH, in is compared with therelevant threshold low level ("TL"). As shown in block 143, if total sumis not less than the threshold low level, the test is exited with nochange in final state. As result of this hysteresis, the device isprovided with an enhanced degree of stability. If total sum TS is indeedless than threshold low level TL, a new state low is indicated as shownin block 144.

The high or low level in the new state thus obtained is next compared tothe previous state. If the test has produced the same result as thatpreviously obtained, new state and old state will be the same. Thiscomparison is illustrated in block 145. If the new state is equal to theold state, a counter may incremented as shown by block 146. Next, asillustrated by block 147, a comparison is performed of the currentcounter value with a predetermined minimum number of cycles in which thestates must correspond in order for a final state change no beindicated. If the counter has not reached the predetermined minimum,then the test exits with no final state change. If, however, the counterreaches the predetermined minimum, then the final state of the test maybe updated to that of the new state. This is indicated in block 148. Adifference in new state and old state, on the other hand, causes thissubroutine to exit without an affirmative change of state. However, asshown by block 149, the new state is substituted for the old state forcomparison during the next cycle. Additionally, the counter illustratedin block 146 is reset to zero.

It can thus be seen than the invention provides a generally noninvasivedevice and method to detect a reciprocative piston located within acylinder bore. The principles and teachings disclosed herein aresusceptible to many variations which may find utility in particularapplications. For example, in some situations it may be desirable toinject two ultrasonic signals of different fundamental frequencies intothe cylinder wall where, in the presence of a piston, mixing would takeplace. One of the transducers could then await arrival of a returnsignal at the sum or difference frequency to indicate a piston presentstate.

In other applications a "trip detector" approach may be used in which apair of ultrasonic transducers are respectively placed at oppositepositions across the cylinder assembly. One transducer could then serveas a transmitter injecting an ultrasonic signal into the cylinder. Ifthe piston is absent, the ultrasonic signal will be allowed to proceedto the other transducer. When the piston is present, however, the signalwill be blocked. Such an absence of a received signal then serves assignal information indicative of piston presence.

Thus, while presently preferred embodiments of the invention andpresently preferred methods of practicing the same have been shown anddescribed, the invention is not limited thereto includes such othervariations as may be desirable in certain applications. It is thereforeto be distinctly understood that the invention is inclusive of suchvariations and any others as may be embodied and practiced within thefull scope of the following claims.

I claim:
 1. A noninvasive proximity detector device operable to detect areciprocative piston located within a cylinder bore defined by acylinder wall, said device comprising:at least one piston sensorpositionable adjacent an outer surface of said cylinder wall at aselected sensing location; said piston sensor including ultrasonictransducer means for selectively generating an ultrasonic signaldirected through said cylinder wall into said cylinder bore; saidultrasonic transducer means further operative after generation of saidultrasonic signal to receive at least a portion of said ultrasonicsignal after propagation thereof through said cylinder wall; and controlmeans responsive to an electrical output of said ultrasonic transducermeans to detect a position of said piston, wherein said control means isoperative to detect a signal level in said at least a portion of saidultrasonic signal after propagation thereof through said cylinder walland at least a portion of said cylinder bore during a sample timeinterval synchronized to have a start time a selected duration aftergeneration of said ultrasonic signal, said sample time intervalcorresponding to period beginning a selected duration after receipt bysaid ultrasonic transducer means of an ultrasonic reflection from aninterface defined by an inner surface of said cylinder wall.
 2. Thenoninvasive proximity detector device of claim 1 wherein said controlmeans is further operative to detect said piston by comparing saidsignal level with a selected threshold level.
 3. The noninvasiveproximity detector device of claim 2 wherein said control means isoperative to derive a final detection signal to be compared with saidselected threshold level by combining a selected number of successivelydetected signal levels.
 4. The noninvasive proximity detector device ofclaim 3 wherein said control means combines by averaging said selectednumber of successively detected signal levels to produce said finaldetection signal.
 5. The noninvasive proximity detector device of claim1 further comprising variable frequency oscillator means for producingan oscillatory output signal having a fundamental frequency controlledby said control means.
 6. The noninvasive proximity detector device ofclaim 5 wherein said fundamental frequency of said oscillatory outputsignal is generally within a range of eight megahertz to twelvemegahertz.
 7. The noninvasive proximity detector device of claim 5further comprising pulse counter means for modulating said oscillatoryoutput signal by a number of driving pulses controlled by said controlmeans.
 8. The noninvasive proximity detector device of claim 7 whereinsaid number of driving pulses is generally in the range of one to sevendriving pulses.
 9. The noninvasive proximity detector device of claim 1wherein said control means comprises:amplifier means for amplifying saidelectrical echo signal by a selected gain; detection signal means forproducing a detection signal during a sample time interval synchronizedto have a start time a selected duration after generation of saidultrasonic signal; and comparator means for comparing said detectionsignal with a selected threshold level.
 10. The noninvasive proximitydetector device of claim 1 wherein said at least one piston sensorcomprises a first and a second piston sensor positionable at first andsecond selected locations respectively adjacent limits of movement ofsaid piston.
 11. The noninvasive proximity switch device of claim 1wherein said control means is operative after generation of saidultrasonic signal to receive an ultrasonic echo resulting fromreflection of a portion of said ultrasonic signal from said piston. 12.A proximity detector device adaptable for use with a cylinder assemblyto detect a reciprocative piston located within a cylinder bore definedby a cylinder wall, said device comprising:ultrasonic transducer meanspositionable on an outside of said cylinder wall at a sensing locationfor selectively generating an ultrasonic signal directed through saidcylinder wall into said cylinder bore; said ultrasonic transducer meansfurther operative to receive said ultrasonic signal after propagationthereof through said cylinder wall to produce a received electricalsignal; and control means initially operative after positioning of saidat least one piston sensor on said cylinder wall for identifying anidentifiable ultrasonic propagation characteristic of said cylinderassembly, said ultrasonic transmission characteristic being atime-of-flight measurement of a duration between generation of saidultrasonic transmission signal and first reception of said receivedultrasonic signal, said time-of-flight measurement indicative of athickness of said cylinder wall, wherein said control means is operativeto produce a detection signal during a sample time interval synchronizedto have a start time a selected duration after generation of saidultrasonic signal, said start time of said at least one sample timeinterval being no less than said time-of-flight measurement.
 13. Theproximity detector device of claim 12 wherein said control meansperforms said time-of-flight measurement by respectively comparing aselected threshold level with a detection signal produced duringsuccessively delayed sample time intervals from a series of ultrasonicsignals generated by said ultrasonic transducer means.
 14. The proximitydetector device of claim 12 wherein said control means is normallyoperative for processing said electrical signal to detect whether saidpiston is in proximity to said piston sensor.
 15. A power actuatedcylinder device having piston proximity detection capability, saiddevice comprising:a cylinder housing having a cylinder wall defining acylinder bore therein and further defining first and second fluidpassages into said cylinder bore; a piston located in said cylinder boreand reciprocatively movable such that expandable fluid chambers aredefined on each side of said piston, said first and second fluidpassages providing ingress of actuating fluid into respective of saidexpandable fluid chambers; at least one piston sensor mounted on anoutside of said cylinder housing at a selected sensing location; saidpiston sensor including ultrasonic transducer means for selectivelygenerating an ultrasonic transmission signal directed through saidcylinder housing into said cylinder bore; said ultrasonic transducermeans responsive to an ultrasonic echo signal produced by reflection ofsaid ultrasonic transmission signal to produce a received electricalsignal; and control means for processing said received electrical signalto detect whether said piston is adjacent said piston sensor, whereinsaid control means is operable to detect whether said piston is adjacentsaid piston by comparing a signal level derived during at least onesample time interval synchronized to have a start time a selectedduration after generation of said ultrasonic transmission signal with aselected threshold level, said control means being operative to beginsaid start time of said sample time interval no less than atime-of-flight between generation of said ultrasonic transmission signaland reception of reflection of said ultrasonic echo signal from aninterface defined by an inner surface of said cylinder wall.
 16. Thepower actuated cylinder device of claim 15 wherein said at least onepiston sensor comprises a first and a second piston sensor respectivelypositionable at first and second selected locations on said outside ofsaid cylinder housing generally adjacent respective terminal limits-ofmovement of said piston.
 17. The power actuated cylinder device of claim15 wherein said ultrasonic transducer means includes a transmittertransducer positioned at a first location on said cylinder housing and areceiver transducer positioned at a second location on said cylinderhousing.
 18. The power actuated cylinder device of claim 15 wherein saidpiston defines a groove about a circumference thereof.
 19. The poweractuated cylinder device of claim 15 wherein said cylinder assembly isconfigured having a mean gap between an inner surface of said cylinderwall and a generally complementary outer surface of said piston suchthat resonant qualities are achieved as said ultrasonic transmissionsignal is reflected from said outer surface of said piston.
 20. Thepower actuated cylinder device of claim 15 wherein a mean gap between aninner surface of said cylinder wall and a generally complementary outersurface of said piston is generally in the range of 0.003 inches to0.023 inches.
 21. The power actuated cylinder device of claim 20 whereina thickness of said cylinder wall is generally in the range of 0.1170inches to 1.04 inches.
 22. A method of detecting presence of a pistonreciprocating within a cylinder bore defined by a cylinder wall, saidmethod comprising the steps of:(a) impressing an ultrasonic transmissionsignal into said cylinder bore from an outside of said cylinder wall;(b) receiving at said outside of said cylinder wall a receivedultrasonic signal produced by said ultrasonic transmission signal; (c)detecting a signal level in said received ultrasonic signal during apreselected sample time interval, said preselected time intervalbeginning a selected duration after receipt of an ultrasonic reflectionfrom an interface defined by an inner surface of said cylinder wall; (d)comparing said signal level with a first selected threshold level; and(e) indicating presence of said piston when said energy level exceedssaid first selected threshold level.
 23. A noninvasive proximitydetector device operable to detect a movable member located within anassembly housing, said device comprising:ultrasonic transmitter meansmounted on an outside of said assembly housing for impressing a seriesof ultrasonic transmission signals into said assembly housing;ultrasonic receiver means mounted on said outside of said assemblyhousing for receiving any ultrasonic signal produced by propagation ofeach of said series of ultrasonic transmission signals through at leasta portion of said assembly housing; said ultrasonic receiver meansoperative to convert ultrasonic signals received by said receiver meansinto corresponding electrical signals; and, control means operativelyconnected to said ultrasonic transmitter means and said ultrasonicreceiver means to determine and update status of presence of saidmovable member based on electrical signals received subsequent togeneration of each of said ultrasonic transmission signals, wherein saidcontrol means is operative to determine presence of said movable memberby comparing a signal level of an electrical signal produced by saidreceiver means during a selected sample time interval with a selectedthreshold level, said sample time interval beginning a selected durationafter receipt of an ultrasonic reflection from an interface defined byan inner surface of said cylinder wall.
 24. The noninvasive proximitydetector device of claim 23 wherein said ultrasonic transmitter meansand said ultrasonic receiver means comprise a single ultrasonictransducer.
 25. A noninvasive proximity detector operable to detect areciprocative piston located within a cylinder bore defined by acylinder wall, said detector comprising:a first and a second pistonsensor positionable at first and second selected locations adjacentrespective limits of movement of said piston; each said piston sensorincluding an ultrasonic transducer selectively generating a ultrasonicsignal directed through said cylinder wall into said cylinder bore; saidultrasonic transducer responsive to a portion of said ultrasonic signalreflected from said piston to produce an analogous electrical signal;control means responsive to said ultrasonic transducer means fordetecting a signal level in said portion of said ultrasonic signalreflected from said piston during a sample time interval correspondingto a period during which said portion of said ultrasonic signal fromsaid piston is expected, said sample time interval beginning a selectedduration after receipt of an ultrasonic reflection from an interfacedefined by an inner surface of said cylinder wall; and said controlmeans further operative to detect said piston by comparing said signallevel with a selected threshold level.
 26. The noninvasive proximitydetector of claim 25 wherein said ultrasonic signal has a fundamentaldriving frequency within a range of eight megahertz to twelve megahertz.